In recent years, in a mobile communication system including cellular phones and base stations, a multi-carrier system that combines and transmits plural carrier signals by applying OFDM (Orthogonal Frequency Division Multiplex) or the like is adopted. In the multi-carrier system, a high peak component occurs in a combined signal at timing when phases of the respective carrier signals overlap. Therefore, in a signal transmitting apparatus that transmits a multi-carrier signal, the signal is supplied to an amplifier after a peak component of the signal is suppressed in advance, thereby the amplifier is actuated near saturation power to improve efficiency of use of power.
As a technique for suppressing a peak component of a signal, there are known, for example, a hard clip method for clipping a signal portion having amplitude larger than a threshold to the threshold and a window function method for multiplying the signal with a coefficient such that the peak component is suppressed to be equal to or lower than the threshold.
FIG. 1 is a graph for explaining a concept of peak suppression by the hard clip method. FIG. 2 is a diagram of a circuit structure for realizing the peak suppressing method illustrated in FIG. 1.
When an original signal A before the peak suppression is inputted to a hard clip section 11 illustrated in FIG. 2, the amplitude |x(t)| of the signal A is calculated and compared with a suppression target value A. When the amplitude |x(t)| of the signal A is smaller than the suppression target value A, the signal A sent from a delay circuit is multiplied with “1”. When the amplitude |x(t)| of the signal A is equal to or larger than the suppression target value A, the signal A sent from the delay circuit is multiplied with a correction amount A/|x(t)|. As a result, in the original signal A before the peak suppression, a portion in which amplitude exceeds the suppression target value A is clipped to the suppression target value A, and a signal B obtained by suppressing a peak component of the signal A is generated.
The hard clip method has an advantage that a peak component of a signal can be easily suppressed by a simple circuit structure. However, a high-frequency component unnecessary for the signal is produced, thereby generating a needless wave to the outside of a band. Therefore, a high-frequency component of the signal B subjected to the peak suppression by the hard clip method is cut by a filtering method.
The signal B inputted to a filtering circuit 12 illustrated in FIG. 2 is multiplied with plural filter coefficients C1, C2, . . . , and Cn. Signals obtained by multiplying the signal B with the plural filter coefficients C1, C2, . . . , and Cn are added up, thereby the high-frequency component is eliminated to generate a signal C.
It is possible to reduce the generation of the unnecessary wave and suppress the peak component of the signal by using both the hard clip method and the filtering method in this way. However, in order to generate the signal C with an unnecessary frequency component eliminated and a transmission signal band component accurately extracted, it is necessary to prepare a multi-dimensional filtering circuit 1B. Therefore, a circuit size and processing time increase.
As a method of preventing the increase in the circuit size and suppressing the peak component, the window function method is widely used (see, for example, Japanese Laid-open Patent Publication No. 2005-20505 and Japanese Laid-open Patent Publication No. 2007-194825).
FIG. 3 is a graph for explaining a concept of peak suppression by the window function method. FIG. 4 is a diagram of a circuit structure for realizing a peak suppressing method illustrated in FIG. 3.
An original signal A before the peak suppression inputted to a window function circuit illustrated in FIG. 4 is conveyed to an amplitude calculating section 21 in which an amplitude value of the signal A is calculated. Subsequently, the amplitude value is conveyed to a peak-position detecting section 22. The peak-position detecting section 22 repeats processing for comparing a tentative maximum value of the amplitude value with the received amplitude value to thereby finally detect a peak position of the signal A. Further, the peak-position detecting section 22 calculates a peak suppression amount on the basis of amplitude in the calculated peak position and a suppression target value A. A suppression-signal generating section 23 selects a window coefficient corresponding to the calculated peak suppression amount out of plural window coefficients stored in a window data memory 23A in advance. A multiplying section 25 multiplies the signal A conveyed from a delaying section 24 with the selected window coefficient to thereby generate a signal D with a peak component suppressed.
However, in the method illustrated in FIGS. 3 and 4, the peak suppression is started after the peak position of the original signal A is detected, resulting in an increase in a delay amount of the signal or an in crease in a buffer capacity for peak suppression processing.